This invention relates to a screw-nut screw coupling with ball circulation, of the type comprising a first and second element mutually coupled via a plurality of balls, in such a manner that a rotary movement of the one leads to a corresponding translatory movement of the other.
In particular, the present invention relates to a screw-nut screw coupling with ball circulation, of the type comprising a first elongated element and a second tubular element substantially coaxial to each other and with an external and an internal helical track respectively, said two tracks being disposed facing each other to define a helical rolling path for a plurality of balls disposed in mutual contact along said path and along a recirculation duct connecting the two ends of said path together.
In couplings of the aforesaid type, a rigid round bar or rod may be used for said first element, comprising an outer helical groove acting as a track for the spheres and constructed by mechanical machining.
In order to obviate the drawbacks and relatively high cost involved in the said mechanical machining, it is known to use for said first element a screw consisting of a rigid core, e.g. a round bar, or a flexible core, e.g. a metal cable, about which a generally metal wire is spirally wound in the form of a spring, in which each pair of adjacent turns defines a portion of said helical track. Finally, by precompressing said spring, it has recently been possible to dispense with the said inner core and construct the said first element solely from a spirally wound metal wire, in which the adjacent turns are disposed in contact with each other.
Constructing the helical track for the spheres by means of a spirally wound metal wire has the disadvantage of allowing only screws with a relatively small pitch to be made, as this latter is a direct function of the diameter of the wire used and may vary from a minimum equal to the diameter of the wire used (when the spring is precompressed) to a maximum generally equal to two or three times said diameter.